I'm not talking about anything you hated with a passion or thought truly damaged a character. Just something you saw no point to, or thought caused more problems than opportunities or seemed too convoluted. Maybe you mildly disliked said event or revelation, but mostly felt like "why?"
For me, Iris West Allen being born in the future. While I'll grant that it was used numerous times (instead of just once and then forgotten), it always felt unnecessary to me and just didn't really add anything to the character, and I didn't enjoy the storylines that utilized it.
For readers of old Impulse comics, I'd like to add giving White Lightning powers. Perhaps I am misremembering and the powers were present earlier, but it seems like she was a totally non-powered human who appealed to teenage boys, then all the sudden she had the power to charm. No point to it, and it made the character a little bit less fun to me.